The role of the pituitary in the diurnal variation in tolerance to painful stimuli and brain enkephalin levels

Studies were carried out on hypophysectomized rats and mice in comparison to sham-operated controls in order to assess the role of the pituitary in the diurnal rhythm in sensitivity to pain, the hyperalgesic effect of naloxone and the effect of stress on brain levels of met-enkephalin. There were no significant differences in jump latencies between hypophysectomized and sham-operated control mice. The jump latencies in the p.m. were significantly greater than those in the a.m. for both the sham and the hypophysectomized mice. In both the sham and hypophysectomized mice and rats, naloxone significantly reduced the jump latencies in the p.m. The stress-induced increase in the p.m. of brain met-enkephalin, furthermore, persisted in the hypophysectomized rats. We conclude that the pituitary is not essential for the diurnal variation in responsivity to pain, the hyperalgesic activity of naloxone or the stress-induced increases in brain met-enkephalin.